1. Technical Field
The present invention relates to a positioning apparatus which uses radio waves from positioning satellites, a control method of the positioning apparatus, a control program for the positioning apparatus, and a computer readable recording medium for storing the control program for the positioning apparatus.
2. Related Art
Conventionally, positioning systems for positioning a current position of each GPS (Global Positioning System) receiver using a satellite navigation system such as GPS have been used.
Such GPS receiver receives a C/A (Clear and Acquisition or Coarse and Access) code, which is one of pseudo random noise codes (hereinafter, referred to as PN (Pseudo random Noise) code), put on radio waves from GPS satellites, based on navigation messages (rough satellite orbit information: almanac, accurate satellite orbit information: ephemeris) which indicate orbits and the like of the GPS satellites. The C/A code is a code serving as a positioning standard.
The GPS receiver then identifies which GPS satellite transmits the C/A code, and calculates the distance (pseudo range) between the GPS satellite and the GPS receiver based on the time at which the C/A code is transmitted and the time at which it is received. The GPS receiver then positions the position of the GPS receiver based on the pseudo range of three or more of GPS satellites and the position of each GPS satellite on satellite orbit (see JP-A-10-339772).
According to the aforementioned GPS receiver, phases of the received C/A codes and replica C/A codes stored in the GPS receiver are aligned with each other, and a position (hereinafter, referred to as code peak position) which indicates the maximum correlation value is thereby calculated.
The aforementioned pseudo range is then calculated using the code peak position.
However, the GPS receiver may receive both satellite radio waves as direct waves and those as indirect waves (hereinafter, referred to as multipaths) which are reflected from a building or the like. If the direct waves and the multipaths are combined, the aforementioned code peak position deviates from a position (hereinafter, referred to as true position) in the case where only direct waves are received. Accordingly, the pseudo range deviates from the true distance. This causes decrease in accuracy of positioning position.
In this case, use of a narrow-correlator (U.S. Pat. No. 5,101,416), for example, allows identification of a code peak position at the true position, despite the effects of the multipaths.
However, since such narrow-correlator uses a wider frequency bandwidth, it is difficult for it to be applied to GPS receivers for receiving weak radio waves.
On the other hand, focusing on the fact that the beginning position of the code peak is fixed even though multipaths are included because the multipaths are always received after the direct waves, a technology to first compensate for noise floors by calculating and integrating (performing time integration of) correlation results sequentially, then identify the code peak beginning position, and calculate the code peak position from the beginning position has been proposed (e.g., U.S. Pat. No. 6,868,110B2).
However, according to the aforementioned prior art, it is necessary to calculate a necessary number of correlation results to compensate for noise floors in the Gauss distribution. Therefore, there is a problem that it is not suitable especially for mobile units, since time for only calculating multiple correlation results is necessary before calculation of the true code peak position, and it takes 10 seconds or more for initialization.